1 Field of the Invention
The present invention relates to a polymer electrolyte fuel cell, in particular, a water supply to a polymer electrolyte membrane of the fuel cell.
2. Related Art
A polymer electrolyte fuel cell is possible to be operated around the atmospheric temperature to accomplish a high energy conversion efficiency and power output efficiency or a cell performance. As a result, the polymer electrolyte has been highlighted recently as a movable power resource or power source of an electric automobile.
The polymer electrolyte fuel cell comprises an electricity generating element constituted by sandwiching a polymer electrolyte membrane having a hydrogen ion conductivity with carbon electrolytes carrying platinum catalyst, or a polymer electrolyte membrane-electrolyte joinder. Then, gas passages are formed across each surface of the electrodes for supplying reaction gases therethrough respectively. The electricity generating element is supported by gas separators at opposite sides to form a laminated structure of the polymer electrolyte fuel cell. Then, a hydrogen gas or fuel gas is supplied to one of the electrodes and an oxygen gas or air, namely, oxidizing gas to the other electrodes to obtain an electro-chemical energy due to a redox reaction of the reaction gases as an electric energy directly. In this case, the hydrogen gas at an anode side is ionized and moved through the polymer electrolyte, on the other hand, the electron is moved to a cathode side through an external load to react with the oxygen to produce a water. Thus, the electric energy due to the electrochemical reaction can be taken out. The hydrogen ion is moved through the polymer electrolyte membrane accompanying a water molecule. Accordingly, if the polymer electrolyte membrane is dried out, the ion conductivity thereof will be remarkably reduced to thereby reduce the energy conversion efficiency. In view of this, in order to maintain a good ion conductivity, it is necessary to supply the water to the polymer electrolyte membrane. To this end, conventionally, there has been provided a humidifier for humidifying the fuel gas and oxidation gas.
Taking reference with FIG. 1, there is shown a conventional fuel cell system 1 schematically.
In the system 1, there is provided a fuel cell stack 2 in which a plurality of the polymer electrolyte fuel cell unit are laminated. To the anode sides of the fuel cell stack 2 is supplied the hydrogen gas as a fuel gas through a supply tube 3. The air as an oxidation gas is supplied to the respective cathode sides of the stack 2 though an air supply tube 4. In order to control pressures of the hydrogen and air , regulators 5 and 6 are provided. Pressure indicators 7 and 8 are provided for detecting the supply pressures. In addition, there are provided hydrogen and air humidifiers 9 and 10 respectively for humidifying the supply gases in the respective gas supply systems so that the supply gases accompanying certain humidity are supplied to the stack 2.
In addition, flow control valves 13 and 14 are provided in air and hydrogen exhaust tubes 11 and 12 from the fuel cell stack 2.
The fuel cell system 1 is provided with a temperature control mechanism for controlling the temperature in the system by means of the supply of a cooling water to the stack 2. The temperature control mechanism includes a heat exchanger 15, cooling water tank 16 and a cooling water circulation pump 17. A thermocouple 19 is disposed in a cooling water circulation line 18 to measure the temperature therein. Meanwhile, when the air is employed for the oxidation gas, an air compressor (not shown) is necessary for pressurizing the air to a desirable pressure.
When the fuel cell is desired to apply for the power source of a vehicle as the automobile, a compact fuel cell system is required to save a space. In this case, it is necessary to provide a compact fuel cell system including the peripheral systems such as the air compressor, and humidifier as a whole.
Therefore, an object of the invention is to provide a compact fuel cell system.
In particular, an object of the invention is to provide a compact fuel cell system as whole by providing a compact humidifier for the oxidation gas.
The above and other objects of the present invention can be accomplished by a polymer electrolyte fuel cell comprising a polymer electrolyte membrane, an anode catalytic electrode disposed at one side of the polymer electrolyte membrane, a fuel gas being supplied to the anode catalytic electrode, a cathode electrode disposed at another side of the polymer electrolyte, an oxidation gas being supplied to the cathode catalytic electrode, control means for controlling a reduction amount of water from the cathode electrode together with the oxidation gas to a sum of a water amount increased at the cathode electrode by being transported from the anode electrode through the polymer electrolyte membrane during a redox reaction of the fuel cell and a water amount produced by an oxidation reaction in the cathode electrode.
Preferably, the polymer electrolyte membrane has a thickness from about 20-80 .mu.m more preferably, about 20-50 .mu.m.
In preferred embodiment, an operating temperature of the fuel cell is maintained from about 50.degree. C. to 80.degree. C., preferably 60.degree. C.-70.degree. C. As a result, the water level at the cathode side can be maintained at a desirable condition.
Preferably, the fuel gas is a hydrogen and the oxidation gas is an air. In the present invention, there is no humidifier for humidifying the oxidation gas, thus, a compact fuel cell system as a whole can be facilitated remarkably.
Preferably, a thickness of the catalyst layer in the electrolyte membrane is less than about 10 .mu.m.
The polymer electrolyte membrane enable protons or hydrogen ions to be transported when it includes a sufficient water so that the an external electric circuit is formed. Thus, the fuel cell which performs an external work can be formed. Namely in order to form the fuel cell, the polymer electrolyte membrane is needed to contain a sufficient water. At the cathode electrode of the polymer electrolyte, the water is produced due to the oxidation reaction. However, if the water product is excessive at the cathode electrode, the output performance of the fuel cell is deteriorated.
Accordingly, in order to maintain a desirable cell performance, the produced water is to be properly excluded from the cathode electrode and thus from the fuel cell system. In short, both a short water condition of the polymer electrolyte and an excessive water condition of the cathode electrode will deteriorate the output performance of the fuel cell system. In addition, the water amount contained in the fuel gas and the oxidation gas it to be maintained properly.
With regard to a water transportation in the polymer electrolyte, there are two types of the water transportation. one is a so called electrical osmotic flux in which a water is transported from the anode to cathode accompanying the proton transportation. The other is a reverse water diffusion flux in which the water is transported from the cathode to the anode. Accordingly, a water balance in the polymer electrolyte depends on the respective amounts of the electrical osmotic flux and the reverse diffusion flux. Generally, in order to balance the electrical osmotic flux with the reverse diffusion flux, it is necessary to humidify the hydrogen gas at the anode electrode to supply a certain amount of water. On the other hand, if the air is employed for the oxidation gas, the air of 2.5 times amount of the hydrogen flux in the anode side stoichiometrically. Thus, if the gas utilization rates are the same for the respective electrodes, the 2.5 times water accompanying the air is taken away from the cathode side compared with the anode side.
At the cathode side, the water is produced by the oxidation reaction and the electrical osmotic flux over the reverse diffusion flux is flew in and increased compared with the anode side. However, the gas flow in the cathode side are greater than the anode side. As a result, a water shortage condition is produced in the cathode side. Conventionally, the humidifier is provided for dealing with this water shortage condition in the cathode side.
The inventors of the present invention found that a desirable output performance of the fuel cell can be maintained regardless of the omission of the air humidifier of the air.
There is shown a model a water transportation on the polymer electrolyte membrane. The amount of the water transportation in the polymer electrolyte membrane is a difference between the electrical osmotic flux and the reverse diffusion flux and thus can be expressed by the following equation. EQU J.sub.M =Si/F (F: Faraday constant) (1)
wherein
J.sub.M : Amount of water transportation through the membrane PA1 S: Net flux of water per a mole of electrons PA1 i: Current density PA1 a: Utilization factor of hydrogen PA1 P.sub.A :Hydrogen supply pressure PA1 P.sub.W(T) :Saturation vapor pressure at temp. of T(.degree. C.) PA1 c: Utilization factor of air PA1 P.sub.C :Air supply pressure.
The water amount produced in the cathode side J.sub.W can be expressed as; EQU J.sub.W =i/2F (2)
Maximum flux of supplied water to the membrane at the anode side J.sub.A(MAX) can be shown by the following equation. EQU J.sub.A(MAX) =(P.sub.W(T) /(P.sub.A -P.sub.W(T)))i/2aF (3)
Wherein
Maximum amount J.sub.C(MAX) of the water amount taken away from the cathode side can accompanying the air J.sub.C can be expressed as follows: EQU J.sub.C(MAX) =(P.sub.W(T) /(P.sub.C -P.sub.W(T)))5i/4cF (4)
Wherein
During the redox reaction of the fuel cell, it is crucial that the sum of the water transportation amount J.sub.M from the anode side to the cathode side through the polymer electrolyte membrane and the water amount produced in the cathode electrode J.sub.W due to the oxidation reaction is balanced with the water amount J.sub.C taken away from the cathode side accompanying the air. It is also crucial that the water amount J.sub.M transported through the polymer electrolyte is balanced with the water supply amount J.sub.A to the anode side.
If the water amount J.sub.C taken away from the cathode is greater than the sum of the water transportation amount J.sub.M and the produced water amount J.sub.W, it is impossible to maintain a desirable water amount at the cathode side. Namely, a dry out phenomenon occurs at the cathode side.
If the water supply amount J.sub.A to the anode side is smaller than the water transportation amount J.sub.M, the dry out phenomenon occurs at the anode side.
In both cases, the cell performance is deteriorated.
The maximum amount J.sub.C(MAX) of the water amount taken away from the cathode side accompanying the air J.sub.C and the maximum flux J.sub.A(MAX) of supplied water J.sub.A to the membrane at the anode side are the saturated vapor water amounts respectively at a temperature. Thus, the maximum amounts J.sub.C(MAX) and J.sub.A(MAX) depend on the temperature and thus as the temperature increases, they increase remarkably. Similarly, the amounts J.sub.C and J.sub.A also increase remarkably as the temperature increases. As a result, where the temperature of the air supply is high, the dry out tends to occur at the cathode side. In order to prevent such dry out, it is desirable to operate the fuel cell at a low temperature so that the humidification amount of the air can be reduced. Where the temperature of the supply gas is low, the supply water amount J.sub.A and the transportation water amount J.sub.M are substantially balanced at the anode side. Where the temperature decreases beyond a certain level, the supply water amount J.sub.A becomes smaller than the water transportation amount J.sub.M and thus the dry out problem occurs.
Meanwhile, as the thickness of the polymer electrolyte membrane is reduced, the reverse diffusion flux is increased. Thus, the transportation water amount is reduced as a whole. It is considered that this is because a gradient of the water density becomes abrupt in the membrane between the anode and cathode sides. Accordingly, it is desirable to reduce the thickness of the membrane in order to prevent the dry out at the anode side which is caused due to the reduction of the water supply amount J.sub.A at the anode side during the low temperature operation.